The present invention concerns electronic control equipment for prior programing of load control conditions by means of an arrangement by which pins are inserted at will into a pin board connection panel and effecting actual control of the loads by means of a servo-amplifier output which depends upon the programmed load control conditions.
Equipment of this kind, known as servo-controllers, is used commonly as electronic control equipment for industrial machinery such as plastic molding machines, industrial robots, machine tools, etc. In order to facilitate understanding of the present invention, it will be explained referring to an example of its application to a plastic molding machine.
Among servo-controllers for plastic molding machines, such an example as that presented in the specification of the U.S. Pat. No. 3,712,772 of Jan. 23, 1973, entitled "Control System for Parison Extruders" and granted to Denes B. Hunkar, is well known. This system has proved quite useful to date. Namely, in this system a parison is extruded by means of an injection cylinder, while an electronic timer is set at a certain time according to the extrusion cycle. In practice, the extrusion cycle is divided automatically into equal sections, so that a pulse may be derived from each of these sections. This pulse, as it is produced, works successively an electronic switch unit consisting of a shift resistor and an electronic switch. This switch unit selects output terminals on a pin board where the wall thickness of the parison is programmed. Therefore, a parison wall thickness signal is extracted successively and is applied to the input of a servo-amplifier. The output from the servo-amplifier is transmitted to a servo-valve, which operates to move a mandrel cylinder. In this case, the motion of the mandrel cylinder is sensed constantly by a differential transformer and the signal is fed back to the input side of the said servo-amplifier. Thus, the mandrel cylinder moves up and down according to the parison wall thickness signal voltage programmed beforehand by means of the pin board and determines the relative motion between an annular die of tubular type and the mandrel.
As stated above, it may be granted that the said control system has yielded an excellent technique as a modern servo-controller. Still, as electronic control equipment for plastic molding machines, a few problems remain to be resolved. The first is that it is difficult to set the parison wall thickness exactly. This is ascribed to the fact that the control is achieved by converting the extrusive motion of injection cylinders into corresponding time units. Namely, as the electronic switch unit carries out a sequence of work successively in response to each of the pulses produced by the electronic timer, the mandrel motion fluctuates accordingly. In this case, however, the extrusive motion of injection cylinders cannot always be kept at a constant speed as it depends both upon the relative position of the mandrel and the die as well as on the type and viscosity of the plastic materials. The injection pressure, therefore, fails to follow the said work sequence precisely and this gives variations in the quantity of the plastic shot, thus making it difficult to set the wall thickness of parisons exactly. The second problem is that, since the extrusive motion of injection cylinders is converted to a time-dependent basis by means of an electronic timer, the time setting of the electronic timer must be adjusted whenever the injection cylinder undergoes a change in extrusion speed or whenever the said control system is applied to a different plastic molding machine. This adjusting operation can be rather troublesome. The third is that, since the programmed instruction signal voltage changes stepwise, it is impossible to set a large change for the parison wall thickness. Of course, this defect may be avoided to some extent by using the CR interpolation system, but practically no effect is to be expected if the cycle of motion is speeded up. The fourth problem is that, since the shift resistor (a ring counter system) incorporated in the electronic switch unit is liable to operating errors due to electromagnetic noise generated in the peripheral equipment, there is a danger of advancing the work sequence unexpectedly.